The condensation reaction of an olefin or a mixture of olefins over an acid catalyst to form higher molecular weight products is a widely used commercial process. This type of condensation reaction is referred to herein as an oligomerisation reaction, and the products are low molecular weight oligomers which are formed by the condensation of up to 12, typically 2, 3 or 4, but up to 5, 6, 7, or even 8 olefin molecules with each other. As used herein, the term ‘oligomerisation’ is used to refer to a process for the formation of oligomers and/or polymers. Low molecular weight olefins (such as propene, 2-methylpropene, 1-butene and 2-butenes, pentenes and hexenes) can be converted by oligomerisation to a product which is comprised of oligomers and which is of value as a high-octane gasoline blending stock and as a starting material for the production of chemical intermediates and end-products. Such chemical intermediates and end-products include alcohols, acids, detergents and esters such as plasticiser esters and synthetic lubricants.
Industrial oligomerisation reactions employing molecular sieve catalysts are generally performed in a plurality of tubular or chamber reactors, similar to those processes employing solid phosphoric acid (SPA) catalysts. With SPA catalysts, the pressure drop over the catalyst bed or beds is increasing gradually over the duration of the run, due to coking and/or swelling of the catalyst pellets, and the reactor run is typically terminated when a maximum allowable pressure drop over the reactor is reached. Molecular sieve catalysts do not show pressure drop increases similar to SPA catalysts. Oligomerisation reactors using molecular sieve catalysts are therefore characterised by longer reactor run lengths, and are typically decommissioned when the catalyst activity has dropped to an unacceptably low level. With these catalysts, the reactor run length that can be achieved is therefore much more sensitive to compounds or impurities in the feed that affect the catalyst activity or deactivate the catalyst, such as catalyst poisons. Strong bases, such as the proton bases or Bronsted bases, are known poisons for the molecular sieve oligomerisation catalysts, which are acidic. Such bases in hydrocarbon streams are often nitrogen containing compounds, such as amines and amides, and they are typically removed from feedstocks for oligomerisation reactions, including those using molecular sieve catalysts. Such organic nitrogen-containing Bronsted bases are characterised by at least one hydrogen atom bound to the nitrogen atom, and are known proton acceptors. Other organic nitrogen components do not have any hydrogen atom bound to the nitrogen, and its nitrogen atom may have three bonds to 1, 2 or 3 surrounding carbon atoms. These nitrogen atoms however still have a free electron pair, and therefore can still act as a base, termed a Lewis base. Lewis bases are known to be much weaker bases as compared to Bronsted bases, and therefore are often ignored or not considered poisons to acid catalysed processes.
Industrial hydrocarbon conversion processes employing molecular sieve catalysts typically run for several weeks before a catalyst change is required or a decommissioning of the reactor is needed. In industrial processes the feeds for the reactions are generally streams derived from catalytic or steam cracking, which may have been subjected to fractionation. The nature of such refining activities is such that there will be variations in the composition of the feed. In addition it may be desired to change the nature of the feed during a reactor run. The catalyst activity and the reaction conditions vary according to the composition of the feed. Furthermore, the reactions are exothermic and the exotherm also depends upon the nature and amount of olefin present in the feed. Isobutylene and propylene are particularly reactive generating a large exotherm.
The feeds that are used for olefin oligomerisation are typically obtained from petroleum refining or petrochemical operations. In particular they are obtained from either the steam cracking or catalytic cracking of streams obtained from the processing of crude oil. The compositions of these oligomerisation feeds depends upon the feed to the cracking process and the cracking conditions that are employed. The composition of the oligomerisation feed and particularly the amount and nature of the impurities in the feed can have a significant impact on the conversion and selectivity of the oligomerisation reaction and can also effect the useful lifetime of the catalyst. Alternatively the feeds may be produced by the conversion of oxygenates such as methanol to olefins.
It is well known that certain impurities such as sulphur containing contaminants and basic nitrogen containing species have an adverse effect on the useful lifetime of the catalyst and processes are employed to remove these contaminants from the feeds.
The present invention is concerned with such processes that employ a molecular sieve, e.g. zeolite oligomerisation catalyst and is particularly concerned with the provision of conditions which enhance the overall conversion and selectivity of the reaction and extend catalyst life. Olefin oligomerisation may be performed in tubular or chamber reactors although the present invention is particularly useful in reactions performed in tubular reactors.
Throughout this application conversion is the percentage of fresh olefin feed that has reacted (and hence not retrieved anymore in the stream(s) leaving the process). It may be determined by making a material balance over the reactor/process and calculating % conversion as 100×(In−Out)/In.
Selectivity is typically defined as the production of the selected desired product(s). On (primarily) C4 feed these are typically the octene molecules, although the dodecenes may be included, and on (primarily) C3 feed these are the hexenes and dodecenes but even more importantly the nonene molecules. On mixed C4/C5 feeds these are the octenes and nonenes (and optionally the decenes), and on mixed C3/C4 feeds these are the hexenes, heptenes, octenes, nonenes (and optionally the decenes). Undesired typically are the heavier oligomers (typically the C10 or C11+ molecules except for the tetramer (mainly C12) made from propylene), and the molecules that are not directly made by oligomerisation of fresh feed olefins, but made via a mechanism involving cracking to other than those fresh feed olefins. On C4 feeds those are typically everything but the octenes. On C3 feed it is other than C6/9/12s. Selectivity is expressed as a % wt found of the desired material relative to the amount of reaction products (excluding unreacted olefins and paraffins).
Throughout an extended production run, the reaction temperature is generally increased to maintain the desired level of conversion and the reaction is typically terminated when a certain temperature representing the limits of the apparatus is required for the desired level of conversion. Catalyst life is expressed as the amount (weight) of oligomer made per amount (weight) of catalyst and provides a value that compensates for throughput variations, and is a result from a material balance over the process throughout the run. The highest temperature that can be tolerated depends upon the equipment and the feed employed, although we prefer to terminate at 300° C. or less to avoid oligomer cracking reactions.
Tubular oligomerisation reactors employing zeolite catalysts typically comprise one or more bundles of tubes also termed “reactor tubes”, mounted, preferably vertically, within a shell. The tubes are packed with the zeolite catalyst typically in the form of pellets and the feed containing olefin reactant is passed through the tubes in which it is oligomerised, typically from top to bottom. The length of the tube in industrial practice is generally from 2 to 15 meters, often from 3 to 14 meters, preferably from 5 to 12 meters, more preferably from 6 to 11 meters, yet more preferably from 8 to 10 meters. The diameter of the tube, the thickness of the walls of the tubes and the materials from which the tubes are made are important, since oligomerisation reactions are exothermic and it is important to dissipate the heat generated by the oligomerisation reaction. Accordingly relatively small diameter, such as an external or outer diameter (OD) from 25 to 75 mm, tubes are preferred, more preferably 35 to 50 mm diameter (OD) tubes. The reactor tubes are preferably of high strength material and are thin walled and of a material with a high thermal conductivity. The high strength is required to withstand the high pressures that are generally used in the oligomerisation of olefins in a tubular reactor employing a zeolite catalyst. Duplex stainless steel is a preferred material for manufacture of the tubes. Higher strength steel and smaller tube diameters allow for smaller wall thicknesses. Duplex stainless steel and a 50.8 mm (2 inch) OD tube allow the wall thickness to be as little as 3 to 4 mm, leaving an internal diameter of the tube of 35-45 mm.
Any convenient number of tubes may be employed in a tubular reactor shell. Typically, operators use from 25 to 500 tubes per shell, arrayed in parallel. Preferred reactors contain about 77 tubes or 180 tubes per shell, although any number may be employed to suit the needs of the operator, e.g. 360 or 420. The tubes are preferably mounted within the shell and a temperature control fluid is provided around the outside of the tubes but within the shell to dissipate heat generated by the exothermic reaction that, in use, takes place within the reactor tubes. One reactor may comprise multiple bundles of tubes, for example up to 7 or 8, or even 9 bundles, and preferably, in use, the temperature of the fluid within the tubes in all the bundles in the same reactor is controlled by means of the same temperature control fluid system. Hot oil or boiling water, under pressure to control the temperature, may be used as the temperature control fluid.
The present invention may also be applied to oligomerisation reactions performed in adiabatic or chamber type reactors. These typically employ a plurality of adiabatic reaction zones in series, with a means of temperature control between the individual reaction zones. In one embodiment, these reaction zones are separate reactors that each contain at least one catalyst bed, and temperature control may then conveniently be accomplished using heat exchangers between the reactors. As an alternative, a chamber reactor may be employed, where several catalyst beds may be provided within one reactor vessel. Temperature control in chamber reactors is more conveniently provided by interbed quench, whereby a fluid that is cooler than the process fluid is injected into the reactor and mixed with the warmer process fluid between two beds. In this way the inlet temperature to the next bed may be controlled by controlling the flow of the quench fluid. The quench fluid typically contains primarily inert components, but may still contain minor amounts of reactants. A most convenient quench fluid in an oligomerisation process is the mixture of unreacted olefins and paraffins that is left over after the reaction, and which is separated from the oligomer product in the stabiliser tower, which is typically but not necessarily the first distillation tower located downstream of the reaction zone. This liquid is available as a cool liquid from the stabiliser overhead condenser and accumulator system. Part of this mixture is typically purged from the process to control the amount of paraffins and other light inerts in the reaction system. Another portion of this stream may be used as diluent for the fresh feed going to the reactor. And a third portion may thus be used for interbed quench, to control the temperature at the inlet of each catalyst bed, the temperature rise over the catalyst bed, and indirectly therefore also the temperature at the outlet of each catalyst bed. Alternatively or in addition, other cool fluids of less reactive composition may be employed as reactor quench, such as selected portions of the oligomer product although that may be less desired.
Historically, oligomerisation reactions over acid catalysts are performed in the presence of water. The light olefinic feedstreams from refinery operations that are used for olefin oligomerisation typically contain water vapour from upstream in the process, because it is either added such as in steamcracking or catalytic cracking, or formed such as in the process of converting oxygenates to olefins. The feedstreams are therefore typically at their water dew point when they are condensed. This water will typically condense together with the light hydrocarbons, and there is usually sufficient water present to form free water that is then separated off by gravity. The liquid hydrocarbon stream containing the olefinic feed for oligomerisation is immiscible with water and has a lower density. It will tend to form a separate liquid layer above any liquid water phase. Due to some water solubility, this layer will contain dissolved water. If a free water phase is formed, the level of dissolved water will be up to the solubility limit of water in the hydrocarbon stream. This limit is different for different hydrocarbon components, and therefore depends on the composition of the hydrocarbon stream.
U.S. Pat. No. 5,672,800 (WO 93/16020) is concerned with the oligomerisation of olefins employing a zeolite catalyst, particularly the zeolite ZSM-22. U.S. Pat. No. 5,672,800 does not indicate the nature of the reactor that was used although it employs small quantities of materials and indicates that under the conditions employed in U.S. Pat. No. 5,672,800 conversion and catalyst life can be improved if the oligomerisation is performed in the presence of water. The compositions in the examples show a significant improvement in catalyst life when water is present. The catalyst life achieved on propylene using the techniques of U.S. Pat. No. 5,672,800 is 1240 weight of oligomer per unit weight of catalyst and 2500 weight of feed per unit weight of catalyst.
The ExxonMobil Olefins to Gasoline (EMOGAS) process was described at the Annual Meeting of the National Petrochemical and Refiners Association, 13 to 15 Mar. 2005, at the Hilton Hotel, San Francisco, Calif., USA. The paper described olefin oligomerisation in a tubular reactor employing a zeolite catalyst and specified that the reaction temperature is controlled with water that is fed on the shell side of the reactor. It is stated that the heat released due to EMOGAS reactions in the tubes evaporates water on the shell side. The temperature profile in the tubular reactor is said to be close to isothermal and the temperature is controlled via the shell side water pressure, which controls the temperature of evaporation, and also by the reactor feed temperature. The tubular reactors are said to usually operate at a pressure between 5.5 and 7.6 MPa (800 and 1100 psi) and temperatures around 204° C. (400° F.).
The EMOGAS brochure also shows Chamber-type reactors using interbed quench for temperature control. Adiabatic reactors in series for oligomerisation using interbed/interreactor cooling for temperature control are discussed in U.S. Pat. Nos. 4,487,985 or 4,788,366 which are silent about water in the feed, but discuss refinery streams as suitable feedstocks that typically are saturated with water. U.S. Pat. No. 4,547,602 discusses such reactors for its second stage (FIG. 3 and column 6), an oligomerisation feed stream that is saturated with water at the temperature of separation (typically a few 100 ppm wt). U.S. Pat. No. 4,456,779 Table I shows a material balance for an oligomerisation process using 3 adiabatic reactors in series, with interreactor cooling. The fresh feed olefins (which are produced by fluid catalytic cracking see column 11, line 33), the oligomerisation reactor feed and its effluent are all shown with a water content of 0.01% mole, i.e. 100 ppm molar.
It has been standard practice to hydrate the feed to oligomerisation reactors in order to prevent excessive temperatures being generated particularly at the start of a reaction run when the feed contacts fresh catalyst and the exotherm is at its highest. As stated previously U.S. Pat. No. 5,672,800 relates to the hydration of olefin feeds to oligomerisation to control temperature and reduce the exotherm. According to U.S. Pat. No. 5,672,800, if the feed has a water content of from 0.05 to 0.25% molar preferably at least 0.06% molar based on the hydrocarbon content of the feedstock, the yields of the desired higher molecular alkene oligomers can be increased and the zeolite catalyst becomes deactivated more slowly. U.S. Pat. No. 5,672,800 specifies that, if the water content is below 0.05 molar %, it should be increased. In Example 1 of U.S. Pat. No. 5,672,800 the moisture content of a feed having an initial water content of 0.02 molar % is hydrated to give a water content of 0.15 molar % and the catalyst life is increased significantly as is the propene conversion. U.S. Pat. No. 6,684,914 also hydrates the olefin feed to at least 0.05 mole % water. International Publication Number WO 2004/009518 suggests that the minimum water content of the hydrated olefin feed should be 0.005 wt %.
Although the use of water has been found to be beneficial, we have found that the water can interact with the zeolite to form oxygenates from the hydrocarbons in the feed. Although the reaction is not fully understood it is believed that some of the olefins in the feed and the water react over the catalyst to form alcohols and ketones which can be converted to acids which have been found to cause severe corrosion in the overhead system of the stabiliser column and associated recycle equipment, which requires equipment replacement and associated down time, and/or the selection of corrosion resistant construction materials. A zeolite catalysed oligomerisation plant is typically equipped with an upstream process step, assuring there is enough water in the feed to the reactor to suit the needs of the catalyst. This is typically in the form of, or combined with, the water wash to remove basic nitrogen compounds from the feeds, and the water wash step is therefore preferentially done using a slightly acidic water stream. Water presence has been known to be beneficial in zeolite-based oligomerisation to control reactivity and typically a minimum level of water in the reactor feed has been proposed.
Water may also be introduced into the feeds during further treating processes, such as for the removal of sulphur. Sulphur removal from such light hydrocarbon streams is typically done by washing with an aqueous solution of an amine, such as mono-ethyl-amine (MEA) or with aqueous caustic soda. Such sulphur removal steps are typically followed by water washing. The wash water is optionally kept slightly acidic, a method which is sometimes used to remove polar and basic nitrogen compounds which can be poisonous for the oligomerisation catalyst, and this also introduces water.
However it has now been found that the presence of water in the oligomerisation reaction on molecular sieve catalysts in both tubular and chamber reactors may lead to the formation of light molecular weight oxygenate compounds, amongst which there are organic acids, which can corrode equipment.
Following the oligomerisation in a tubular or a chamber reactor the product is typically passed to a stabiliser separator which is sometimes known as a drum or tower where the product is separated into the desired oligomer and the unreacted material and reaction byproducts.
The stabiliser is usually a distillation tower in which the desired oligomer is recovered as the bottoms product, while the unreacted material is taken off overhead. Any water—and oxygenates with low carbon number present in the reactor product—will move up the tower with the vapour, and mostly condense in the tower overhead condenser, together with most if not all of the hydrocarbon lights, such as the unreacted olefins and alkanes. With partial condensers, there typically remains a—much smaller—vapour stream that is not condensed but disposed of as vapour, typically by letting it down into the site fuel system to recover its heating value. The condensing of water with low carbon number oxygenates, in particular the acids, has been found to create corrosion problems in the overhead system of the stabiliser tower.
Part of the condensed hydrocarbon lights are pumped into the stabiliser tower to provide reflux in the tower and the rest is either purged from the system (e.g. routed to the LPG pool or sold as it is as LPG, i.e. “Liquified Petroleum Gas”) or partly used as a diluent recycle to the oligomerisation unit. The recycle may be returned to the feed in order to control olefin strength of the stream going to the reactor. In chamber plants parts of this recycle may be used as interbed quench for temperature control of the reaction in the individual reactor beds. The presence of the oxygenates and acids in the recycle streams can cause corrosion problems in the recycle systems and in the systems receiving the stabiliser overhead streams.
The reaction effluent (after typically some cooling and reduction of pressure) may flow into a splitter tower first, which optionally is preceded by a flash drum, although typically both the vapour and the liquid from the flash drum flow into the splitter tower albeit usually at different levels in the tower. The splitter tower overhead stream then passes to the stabiliser, and the stabiliser bottom stream then only contains part of the polymer, i.e. the lighter oligomers. Some of the lighter oligomers, such as hexenes and heptenes may be recovered from the stabiliser tower bottoms by further fractionation in a “light product” unit, and the remaining stream containing heavier oligomers may be routed for further recovery to a “heavier product unit”. The splitter tower bottom stream may contain very little of the lighter oligomers and it may be taken to a “rerun tower” where the very heavy oligomer is retained in the bottom. The “intermediate” oligomers taken overhead may be combined with the heavies from the light product unit for recovery of octenes, nonenes, decenes, undecenes and/or dodecenes by further fractionation in the “heavier product unit”.
Reference in this specification to removal of heat from the (reactor) tubes of tubular reactors or temperature control of the (reactor) tubes is, in context, intended to mean removal of heat from the materials contained within the tubes where reaction takes place (generally comprising, in use, unreacted feed, reaction products and catalyst). It will be appreciated that the heat generation on the catalyst and heat removal from the tube wall may cause a radial temperature gradient through the cross-section of the tube, such that the centre of the tube may become significantly hotter than the wall of the tube. The larger the tube diameter, the larger this temperature gradient may be. One convenient way to remove the heat from the tubes and carry out the temperature control is to provide boiling water to generate steam within the reactor on the shell side around the exterior of the tubes. This provides a good heat transfer coefficient on the shell side. If the present invention is performed in a chemical plant or a refinery, the steam generated by the oligomerisation process may be readily integrated into the steam system typically present at such sites. The reaction heat from oligomerisation may then be put to use in another part of the oligomerisation process, or with another process in the plant or the refinery, where heat input is required.
In adiabatic type reactors, the highest temperatures occur at the outlet of the reactor beds. These temperatures may be controlled by controlling the inlet temperature to the corresponding reactor bed, either by interbed cooling or by interbed quench, and by reactor inlet temperature control for the first bed.
On an industrial scale it is desirable that oligomerisation reactors can run continuously for as long as possible (i.e. long catalyst life) and that the conversion and selectivity of the reaction is maintained over such extended production runs.
As already indicated, the oligomerisation of olefins over zeolite catalyst is a highly exothermic reaction, particularly the oligomerisation of propylene and/or isobutylene. The high temperatures generated by the exotherm can lead to carbonaceous deposits on the catalyst caused by a build up of condensed, heavy hydrocarbons similar to asphalt. Such deposits are commonly termed “coke” and, may occur inside the zeolite or molecular sieve pores and/or on the outer surface. This coke formation can lead to deactivation of the zeolite catalyst. In general, the higher the concentration of olefin in the feed, the higher will be the rate of heat release from the catalysed reaction, and hence the higher the temperatures reached. Consequently there will be a higher rate of coke formation. This has placed a limit on the maximum concentration of olefin that can be tolerated in the feed. Since the oligomerisation reaction is highly exothermic it is necessary to control the temperature and in a tubular reactor this is usually accomplished by encompassing a bundle of reactor tubes within a shell through which is passed a temperature control fluid. Conveniently the temperature control fluid is oil (usually hot oil), or preferentially a boiling liquid because of the improved heat transfer on the side of the boiling liquid. This boiling liquid may be an organic stream, preferentially a stream taken from another point in the process and its return stream, usually a mix of vapor and liquid, returned to another suitable point in the process. The reaction heat may as such be used as heat supply to a reboiler of a distillation tower. Most conveniently the liquid is water, at least partially converting to steam in the reactor shell side. The water is conveniently supplied from a steam drum and the boiling temperature can then readily be controlled by varying the pressure in the steam drum. Conveniently the steam drum collects the water/steam return stream from the reactor shell side, and provides the water supplied to the reactor shell side. The steam generated by the reaction heat may be removed from the steam drum and may be put to use elsewhere.
In adiabatic reactors such as chamber reactors, as in tubular reactors, coke buildup on the zeolite catalyst will be highest at the location of the higher temperatures, which in adiabatic reactors occur at the bed outlets. In adiabatic type reactors it is therefore particularly important to control the bed outlet temperatures such as by the use of a quench fluid as mentioned previously. The use of a quench fluid brings the additional benefit over a conventional heat exchanger, in that it tends to dilute the reactants in the process fluid, so that the same amount of reaction heat generated will cause a lower temperature rise of the stream passing through the catalyst bed.
Traditionally water has been provided in the feed as an additional means to control the temperature of the reaction and to compensate for the exotherm.